Movie Math: "The Fugitive" sherriff should have had better math skills!

I was watching the movie, "The Fugitive" starring Harrison Ford when I heard something that gave me pause, mathematically speaking. After Richard Kimble escaped the first time by way of the bus/train accident, the small town sheriff was relieved of the case by another official (who, by the way, referred to the sheriff as "Wyatt Earp" because of how he was handling the situation ~ not nice!) The relieving officer, debriefing those who were assisting, claimed that Kimble had been running for about 90 minutes at a foot speed of about 4 miles per hour. Therefore, they had about a 6 mile radius. Was this an accurate, overstatement or understatement of the situation?